Catalytic converters are employed on motor vehicles to control atmospheric pollution. A catalytic converter typically consists of a ceramic or metallic monolithic substrate mounted within a metal housing. The ceramic or metallic monolithic substrate has a catalyst washcoat comprising a catalytic metal disposed on an inorganic oxide support. Preparation typically comprises coating a washcoat onto the exposed surfaces of the monolith. The washcoat is dried and the washcoated, dried, monolith substrate is calcined. The catalyst is responsible for the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. These catalyst compositions have a very long service life without deactivation.
The improvement of engine efficiency from vehicles is a worldwide goal. Lean burn, high air-to-fuel ratio and diesel engines are certain to become more important in meeting the mandated fuel economy requirements of next generation vehicles. However, high NOx emissions from high air-to-fuel ratio diesel and lean burn engines cause significant environmental problems. Accordingly, development of effective and durable catalysts for controlling NOx emissions under net oxidizing conditions is critical.
This ongoing effort to reduce emissions creates challenges for the emissions design engineer. Catalysts of various types have been employed by themselves for the reduction of automobile emissions and have been effective in meeting the standards of the past. However, ever tightening regulations have made necessary a more complex system for controlling emissions. Catalysts are located closer to the engine for faster catalyst activity. Those catalysts are increasingly exposed to temperatures as high as 1200° C., and therefore excellent heat tolerance is required.
High catalyst bed temperatures cause complete hydrocarbon combustion wasting valuable species that could be used for NOx reduction. Reforming of the hydrocarbon portion into highly active reducing agents is necessary if high air-to-fuel ratio engines are to be increasingly used. Nanoscale sized catalyst constituents (that is, catalysts having particles of about 2 to about 80 nanometers, typically less than about 70 nanometers) are necessary for high efficiency fuel reforming and are therefore gaining in importance.
Another disadvantage is that the temperatures required for solid materials to react can be high enough that the reacted materials only form phases having a low surface area and therefore exhibit low catalytic activity.
Conventional precipitation methods yield products where one material substantially precipitates while a second material barely precipitates. Instead of a homogenous powder of a single material resulting, a simple physical mixture of two different materials results. Calcined autoclave generated powders often consist of large agglomerates of non-homogeneous catalyst particles.
In addition, conventional convection drying of catalyst materials dries the catalyst from the outside in. This causes the platinum group metals to migrate from the interior to the exterior surface of the inorganic oxide support structure. Platinum group metals at exposed surfaces are easily poisoned and quickly agglomerate into larger, less active particles.
When a washcoated substrate is impregnated with a platinum group metal solution, conventional drying (oven drying) causes the platinum group metals to migrate towards the outer regions of the substrate. Unfortunately, the exhaust gas flows primarily through the center of the substrate.
The moisture content of a washcoated substrate essentially drops below about 1 weight % of the total washcoat weight when the drying process ends and the calcinations process begins. Soluble species in the washcoat slurry are capable of plugging the inorganic oxide support porosity, particularly critical mesopores of about 10 to about 100 nanometers (average diameter). With conventional convection drying which dries from the outside in, species in solution migrate from the wet interior surfaces towards the dry exterior surfaces. As such, conventional convection dried washcoats have soluble binder migration towards the washcoated surface, increasing the washcoat density and plugged porosity at the washcoat surface relative to the washcoat subsurface.
There have been thousands of computer modeling studies of automotive exhaust systems. Still, today converters used in production are basically the same shapes and are located in about the same positions as first developed in 1972. Improvements in conversion have been largely due to better materials.
One useful piece of information gained from computer modeling is that the flow distribution through a substrate is not uniform. Most of the flow is through the center. As a result, all sorts of flow diverter valves, etc., have been designed to force the flow through the outer portion of the substrate. However, due to complications of implementation, as far as the present inventors are aware, no device has ever been adopted.
Catalyst materials and manufacturing development have not included changes that take advantage of these flow patterns. In fact, previous metals gradients in catalyst materials are directly opposite of the desired gradient. As a result of low precious metal deposition in the center of a substrate, precious metal concentrations must be increased such that the low concentration center has a sufficient concentration for the high quantity of exhaust flow. The precious metals deposited in high concentration at the edges of a substrate are wasted. The resultant increase in precious metal costs are a great disadvantage to manufacturers.
Catalytic reactions are time dependent. The faster exhaust gasses travel through a channel, the more active the catalyst must be. NOx reduction is particularly sensitive to residence time. To achieve high NOx efficiency, the catalyst must be sized large enough that the necessary residence time is obtained even at maximum exhaust flow rates.
When determining the proper sizing, it must be taken into account exhaust gas flow is not the same through all the substrate cells. Greater than about 65% of the exhaust gas flows through the cells in the substrate center which encompasses an area not more than about two inches in diameter. Much less exhaust gas flows through the substrate edges. The residence time in those center cells must be long enough to achieve the desired NOx reduction.
The precious metal concentration at the center cells of the substrate can be increased to achieve the desired catalytic reactions. However, since the precious metals are in the slurry, the high concentration is deposited over the entire wet washcoated substrate. The high concentrations of precious metals at the outer portion of the substrate are wasted. NOx reduction is accomplished on a rhodium surface. Rhodium is the most expensive of the precious metals used, and therefore the element we least desire to waste.
The wet washcoated substrates must be calcined. The most practical method of substrate calcinations involves placing the wet washcoated substrate on a belt furnace. As the belt moves forward and the substrate moves towards the “hot” zone, moisture evaporates from the exposed substrate surface. As the surface dries, the moisture inside the substrate migrates towards the dry substrate skin. Since the precious metal salts are water-soluble, the precious metals migrate with the moisture towards the outer substrate cells. As a result, the center cells of the calcined substrate end up with the lowest concentration of precious metals. The outer cells of the calcined substrate end up with the highest concentration of precious metals. The result is a distribution that requires the highest precious metal loading for a given NOx conversion.
One method used to correct for this unwanted distribution is use of a substrate shaped like a bullet, for example, configuring the center cells at about six inches long and the edge cells at about three inches long. Bullet nose substrates have about the same exhaust gas residence time in all of the substrate cells. Therefore, all the precious metal can be efficiently used. A bullet nose substrate with about 30% to about 40% less precious metals has the same NOx activity as a standard substrate.
The durability of the substrate is also a significant issue. The inlet face of a substrate tends to have erosion and breakage during the current useful life emission standard. In 2004, emission standards increased the useful life to 120,000 miles or 11 years. Further, if a vehicle manufacturer extends the useful life to 150,000 or 15 years, the manufacturer can receive additional NOx credits.
The current NOx standard for an ultra low emissions vehicle (ULEV) is 0.3 grams per mile. The 2004 NOx standard for a ULEV vehicle is 0.07 grams per mile. The new SULEV category reduces the NOx standard to 0.02 grams per mile. A significant increase in NOx reduction activity is required. One way to achieve such an increase is by adding more precious metals and more washcoat. This, however, means a significant increase in expense due to the high cost of precious metals.
To increase the exhaust gas residence time, the cell density can be increased from about 400 cells per square inch to about 600 or about 900 cells per square inch, and the cell wall thickness can be reduced from about 8 mil walls to about 2 mil walls. However, to extrude a higher cell density substrate with thinner walls, the substrate formulation has to contain less clay binder and more zirconium oxide. The resulting cell walls are stiffer, but not as strong.
Further, the cost of thin walled substrate is several times more expensive than the cost of a standard substrate. Also, the standard mat material loses mica, and the loose mica plugs the substrate face. Therefore, a ceramic alumina fiber mat must be used with thin walled substrates. The alumina mat exerts less peak pressure on the substrate and there is no mica in the alumina mat. Unfortunately, the alumina fiber mat can be more than double the cost of the intumescent mica mat.
Further, a standard converter has the full length of mat material to retain the substrate in the exhaust. The full length of “gripping” allows the force per unit area to be low enough that breakage during “stuffing” does not occur. In comparison, bullet nosed substrates have only half of the “gripped” area of standard shaped substrates. The pressure applied by the alumina mat must be increased to compensate. Thin walled substrates with increased mat density and lower “gripped” area present a processing challenge.
The shape of bullet nosed converters exposes many edges of the cell walls. The exposed cell walls easily fracture. Broken pieces of substrate tend to plug the inlet face of the substrate. Higher cell density substrates with thinner, stiffer walls will not meet durability requirements.
Finally, metal monolith substrates are made of very thin foils of about 0.02 to about 0.03 millimeters thickness. Bullet nosed substrates cannot be made from thin metal monolith foil because the edges fold over, resulting in a plugged frontal area.
U.S. Published Patent Application 20040077911 entitled “Lithium aluminate layered catalyst and a selective oxidation process using the catalyst” discloses a catalyst for the selective oxidation of hydrogen. The catalyst comprises an inert core such as cordierite and an outer layer comprising a lithium aluminate support. The forming process comprises drying a support layered with catalyst at a temperature of about 100° C. to about 350° C. followed by calcination at a temperature of about 400° C. to about 1300° C. In forming the lithium aluminate support, the alumina layered composition is impregnated with a lithium compound. The support is immersed in an impregnating solution and the resultant composite is allowed to dry under ambient temperatures or is dried at a temperature of about 80° C. to about 110° C. followed by calcination at a temperature of about 400° C. to about 300° C. thereby forming lithium aluminate. The disclosed method is typical of currently available catalyst preparation processes employing conventional drying for both drying and calcination steps.
In art areas not related to exhaust catalyst preparation, various methods for preparing ceramic honeycomb bodies include drying systems employing microwave drying and conventional oven drying. U.S. Published Patent Application 20020109269A1 entitled “Method of fabricating honeycomb body and drying system” discloses a method of fabricating at least a honeycomb body and a drying system. A honeycomb mold having a cell wall thickness of not larger than 0.125 mm can be dried without developing any cracking or wrinkles in the outer peripheral skin portion. In a method of fabricating a honeycomb mold (1) of ceramics having a multiplicity of cells (10) defined by the cell walls (11) having a thickness of not more than 0.125 mm arranged in the shape of honeycomb, each extrusion-molded argillaceous honeycomb body (1) is dried by being exposed to a high-humidity ambience of 70% or more while at the same time being irradiated with microwaves in the frequency range of 1,000 to 10,000 MHz.
U.S. Pat. No. 6,455,826 entitled “Apparatus and method for continuous microwave drying of ceramics” discloses an apparatus capable of continuous drying of ceramic articles which produces little or no microwave radiation emission. The drying apparatus comprises a microwave-heating chamber for heating a ceramic, having an entrance and an exit end and a material flow axis along which the ceramic articles are conveyed. Positioned adjacent the entrance and exit ends of the microwave-heating chamber, respectively, are a first and second attenuation chamber each having an entrance and an exit end. An inlet chamber, having a material flow path, is connected to the entrance end of the first attenuation chamber with a portion of the material flow path disposed at an angle to the flow axis. Connected to the exit end of the second attenuation chamber is an outlet chamber having a second material flow path; again at least a portion of the material flow path is at an angle to the material flow axis.
U.S. Pat. No. 6,344,635 entitled “Hybrid method for firing ceramics” discloses a method of firing ceramic materials involving placing the ceramic material in a microwave heating apparatus having a microwave cavity and subjecting the ceramic material to a combination of microwave radiation and conventional heat energy according to a predetermined time-temperature profile. The time-temperature profile, ranging from room temperature to sintering soak temperature, comprises a series of target heating rate temperature set points and a series of corresponding core and surface temperature set points with each of the core and surface temperature set points being offset from the target heating rate set points a predetermined offset temperature.
The disclosures of each of the foregoing U.S. Patents and U.S. Patent Applications are hereby totally incorporated herein by reference in their entireties. The appropriate components and process aspects of each of the foregoing U.S. Patents and U.S. Patent Applications may be selected for the present method and system in embodiments thereof.
A need remains in the art for an improved catalytic exhaust treatment device for sustained high temperature operation, i.e., temperatures greater than or equal to about 1050° C. There further remains a need for an improved method for preparing such a device.